This invention relates to a method for the selective scission of the carbon-to-carbon bond of the ethyl group of ethyl aromatics to produce methyl aromatics. Alkyl aromatic compounds have long been produced from hydrocarbon fractions relatively rich in such materials. Early sources were liquids from coking or other distillation of coals. More recently, these products have been derived from fractions obtained in refining of petroleum. An important source in recent years has been the aromatic liquid naphthas resulting from the thermal cracking of gases and naphthas to produce olefins.
However derived, these aromatic-rich streams containing a broad range of components have usually been distilled and otherwise separated (e.g. solvent extraction) to obtain the desired product components. The purpose of those operations typically has been to obtain paraxylene and benzene which are now used in huge quantities in the manufacture of terephthalic acid and other chemical products. The separated streams resulting from the above separation by distillation or other means accordingly consist of product streams of benzene, toluene, C.sub.8 aromatics containing xylenes and a bottoms product of C.sub.9 aromatics containing ethyltoluenes, trimethylbenzenes and C.sub.10 + aromatics. The C.sub.9 component can be separated by means of distillation and can be a source material for manufacture of lighter aromatic hydrocarbons by hydrocracking but with some attendant material losses. The C.sub.10 component is useful for heavy solvents and gasoline.
The transalkylation of toluene and C.sub.9 aromatics including trimethylbenzenes has been widely studied (U.S. Pat. Nos. 3,260,764; 3,527,825; 3,677,973) because of the demand for greater quantities of high purity aromatic hydrocarbons but the results of such studies have not been sufficient to cause supplies of these hydrocarbons to increase sufficiently to meet this demand. One of the sources of C.sub.9 aromatics can be the heavy reformate stream obtained in refining of petroleum; however, the trimethylbenzene concentration in heavy-reformate derived C.sub.9 aromatics often is only 50-60%. The remaining C.sub.9 aromatics content can consist of 35-42% ethyltoluenes and 6-10% propylbenzenes and indan. The presence of ethyltoluenes and propylbenzenes in a transalkylation reaction feed can have a detrimental effect on both xylene yield and quality, because they can contribute to the formation of undesirable by-products such as ethylxylenes and ethylbenzenes. Accordingly, it is advantageous to remove the ethyltoluenes from reformate-derived C.sub.9 aromatics, preferably by converting them to useful compounds such as xylenes. This would provide also a trimethylbenzene-rich stream which is ideally suitable as transalkylation feedstock.
This invention relates to an ethyl scission process for the selective conversion of fractionated heavy reformate comprising ethyltoluenes into more useful compounds. More specifically, this invention is concerned with a selective scission process whereby a fractionated heavy reformate stream comprising ethyltoluenes is converted primarily to xylenes while formation of benzene is minimized, utilizing a catalyst comprising a metal selected from Group VIII metals and Group VIII metals promoted with zinc on a carrier of highly purified gamma alumina containing essentially no silica and of a surface area of at least 100 m.sup.2 /g. Fractionated heavy reformates are reformates from which C.sub.8 aromatics and lighter components have been largely removed. This stream typically contains C.sub.9 aromatics consisting primarily of trimethylbenzenes and ethyltoluenes in a respective ratio of approximately 1.8:1. Ethyl scission of the ethyltoluenes contained in this fraction to xylenes can provide a trimethylbenzene-rich C.sub.9 aromatic stream which is ideally suited for preparing additional xylenes via transalkylation with toluene.
In the prior art, methods, which have been used to produce aromatic chemicals from fractionated heavy reformates, can utilize a hydrocracking and/or a hydrodealkylation step to convert the C.sub.9 and C.sub.10 + aromatic components to benzene, toluene and C.sub.8 aromatics. The C.sub.6 + paraffins are converted into readily distillable low boiling hydrocarbons of C.sub.5 and lighter. Processes utilizing these principles are described in U.S. Pat. Nos. 3,957,621 and 3,862,254.
Extensive experiments in the prior art on hydrodealkylation of pure methylethyl and trimethylbenzenes and of mixed alkylbenzene feeds obtained from heavy reformate in the presence of certain catalysts have demonstrated the effects of the nature of the feed and catalyst in the yields and ratios of the xylene isomers produced. For example, alkalized cobalt molybdate catalyst favored p-xylene whereas nickel catalyst produced high percentages of o-xylene with extensive hydrogenolysis of the aromatic ring. Riv. Combust. 20 No. 1:3-35 (Jan 1966).
Other typical prior art on hydrodealkylation of alkyl aromatics is the following:
U.S. Pat. No. 2,422,673 teaches hydrodealkylation or demethylation of an alkyl aromatic using a catalyst containing nickel or cobalt on diatomaceous earth. Temperatures used in the process are between 350-650.degree. F. and pressures are between subatmospheric to 1000 psig. The reaction is carried out at a low pressure of hydrogen so as to obtain a high proportion of demethylation and a relatively small amount of hydrogenation of aromatic hydrocarbons to naphthenic hydrocarbons.
U.S. Pat. No. 2,734,929 teaches hydrodealkylation of alkyl aromatics. Ethylbenzene is hydrocracked to principally benzene and xylene to toluene. The catalyst contains a Group VIB or Group VIII metal hydrogenation component such as chromium, molybdenum, tungsten, iron, cobalt, ruthenium, rhodium, etc. The catalyst is preferably suspended on a carrier which has no adverse effect on the reaction. Gel alumina, which contains silica and which usually has a surface area of over 100 m.sup.2 /g, as measured by gas adsorption, is preferred. The process requires a gaseous diluent which, column 2, lines 16-18, is stated as being a critical feature of the invention. Operating conditions include a temperature between 800.degree.-1500.degree. F. and a pressure of 0-5000 psig.
U.S. Pat. No. 3,478,120 discloses a process for hydrodealkylation of ethylbenzene to toluene, benzene, methane and ethane with the hydrodealkylation being carried out in the presence of xylenes. The catalyst used comprises an iron group metal on calcium aluminate. Operating conditions include a temperature range of 500.degree.-1200.degree. F. and pressure from atmospheric to 2000 psig.
U.S. Pat. No. 3,306,944 teaches a catalytic hydrodealkylation of alkyl aromatic hydrocarbons. Examples are cumene to ethylbenzene, predominantly, and toluene; p-t-butyltoluene to p-propyltoluene, p-ethyltoluene and xylene. The catalyst comprises a metal selected from the group consisting of rhodium, ruthenium, etc. upon a promoted metal oxide support. "Promoted" refers to a pretreatment of the support with a salt or hydroxide of an alkali metal or alkaline earth metal. The preferred metal oxide support is gamma alumina which has a surface area ranging from 100 to about 300 m.sup.2 /g and is freed from combined or adsorbed water.
U.S. Pat. No. 3,992,468 teaches a catalytic hydrodealkation process of alkyl aromatic hydrocarbons to benzene. The catalyst comprises at least two metals, one selected from the group consisting of ruthenium, cobalt, osmium, palladium, rhodium, iridium, platinum, chromium, molybdenum, tungsten and manganese, the other selected from, among others, zinc, cadmium, and gallium, the final catalyst having a specific surface area of from 1 to 100 m.sup.2 /g. The carrier is of low acidity and can be alumina, including gamma alumina, magnesia, magnesia-silica, acidic alumina, alumina-silica, among others, including molecular sieves.
U.S. Pat. No. 3,975,454 teaches a catalytic hydrodealkylation process of alkyl aromatic hydrocarbons at a temperature within the range of 250.degree.-400.degree. C. The catalyst comprises the compounds formed from either graphite and an alkali metal, or graphite, an alkali metal and a compound of a metal selected from the group consisting of Group VIII of the Periodic Table which includes iron, nickel, cobalt, etc. Surface area of the catalyst of Example I was cited as about 20 m.sup.2 /g.
Accordingly, the prior art teaches catalytic hydrocracking and/or hydrodealkylation of alkyl aromatic hydrocarbons. However, the catalytic ethyl scission of alkyl aromatic hydrocarbons is not taught wherein the hydrocarbon comprises ethyltoluenes and the catalyst used comprises a metal selected from the group consisting of a Group VIII metal and a Group VIII metal promoted with zinc upon a high-surface area carrier of highly purified gamma alumina containing essentially no silica, with a surface area greater than 100 m.sup.2 /g under process conditions which selectively convert ethyltoluenes to predominantly xylenes.